The present invention relates to a method of drawing seamless metal tubing, particularly copper tubing or pipes under utilization of a floating mandrel being disposed and working inside the hollow as it is being drawn and under further utilization of a die which reduces the outer diameter of the tubing pursuant to the drawing and wherein the wall thickness of the hollow being drawn is reduced under cooperation of the mandrel and the die while a drawing force is applied to the tubing as it has been drawn downstream from the die.
The modern manufacture of seamless copper tubing follows basically the procedure outlined above whereby particularly a solid block is first converted e.g. through hot rolling or press working into a hollow and this hollow is then reduced as far as wall thickness as well as outer diameter is concerned e.g. by means of a cold working pilgrim step like mill. Thereafter the tubing is drawn in several stages under utilization of dies and mandrels to obtain the final dimension. The tubing made is an intermediate product by the pilgrim step method is prepared in that a mandrel as well as drawing oil is introduced in the tubing, and then the end of the tubing is narrowed or tapered so that in fact one can "thread in" the tubing, into and through the drawing die. The preparation for the next drawing step is similar whereby it might be advantageous to have cut off the narrowed point or tapered end of the tubing as per the previous step.
All this procedure is rather time consuming and cumbersome and to some extent wastefall. Many tests have been conducted to reduce the preparatory steps and surrounding work procedure. It has been suggested for instance to provide the pointed or tapered end of the tubing in such a manner that the same tape end can be used for threading the tubing into several dies for sequential drawing passes. Another method lead to the development of preparing the length of the tubing during a period of time while the preceding tubing is still being drawn and processed.
Unfortunately it was found that all these method and improvements are disadvantaged by very high expenses in machinery and equipment. It has also been suggested to provide thick wall tubing and pipes in such a manner that certain short sections are made initially by press working or rolling and they are interconnected through end-to-end or butt welding so that one obtains a rather long string of tubular length for further procedure. Here then a short tube section is rotated fast and welded at the end in this kind of preparatory process.
However, it was found that the welding seams made in such a manner do not have the adequate strength in many cases so that a string of tubing composed of these welded short sections cannot really be pulled through the die without incurring further problems because the mandrel--die cooperation is not in a position to provide the requisite forces owing to the weakness of these various welding seams.